Radiation therapy (RT) is a popular and efficient method for cancer treatment, where ionizing radiation is used in an attempt to kill malignant tumor cells or to slow down their growth. RT is often combined with surgery, chemotherapy, or hormone therapy, but may also be used as a primary therapy mode. Radiation therapy may be administered as internal RT or brachytherapy or, more commonly, external beam RT.
Internal RT treatment typically includes placing one or more radioactive sources near a designated treatment area, either permanently or temporarily. Conversely, external beam RT typically involves directing radiation beams produced by sources located externally with respect to the patient or radiation subject to the afflicted treatment area. The beam can consist of photons, electrons, protons or other heavy ions; photons being (at present) the most commonly used particle type. Malignant cells are damaged by the ionizing radiation used during the RT. However, the damage from the radiation is not limited to malignant cells and thus, the dosage of radiation to healthy tissues outside the treatment volume is ideally minimized to avoid being similarly damaged.
The development of medical linear accelerators (linacs) have dramatically increased the practicality and efficiency of multi-field RT treatments. Even more recently, linacs have been equipped with specialized computer-controlled hardware devices, such as collimator jaws and multi-leaf collimators (MLCs). These devices have been developed to deliver fields conforming to the projection of the target with greater ease and accuracy. In more advanced applications, the collimator jaws and/or the individual leaves of an MLC are moved separately under computerized control systems at desired speeds during periods of radiation (e.g., beam-on). This has enabled the generation of spatially modulated radiation fields, since each leaf attenuates the beam for a different time period. The resulting intensity modulated radiotherapy (IMRT) has allowed the application of high dose volumes that conform more closely to the shape of complicated targets. The further integration of x-ray image receptors to the linac has enabled the imaging of the patient before each treatment session and the tracking of tumor motion during treatment delivery. These so-called image-guided RT methods have improved subject positioning accuracy, and have lead to techniques for restricting tumor motion during treatment.
However, while these developments allow programming of more accurate beam fields, the devices themselves are still subject to mechanical errors or measurement variances which may result in inaccuracies during radiation application. Traditionally, verification of the mechanical devices were performed by generating a sample image under certain pre-programmed conditions or with pre-defined parameters. The mechanical devices could be taken apart and measured, and the generated image would be manually measured to verify the positioning of the collimator components under these known conditions. However, such verification techniques can be extremely time and user intensive, require significant skill to perform, and are subject to user error.